Electronic fingerprint sensing has received increased attention as a technique for reliable identification of individuals. Electronic fingerprint sensing may be used in stationary equipment, such as security checkpoints, or in portable devices, such as mobile phones and other wireless devices, and smart cards. Accordingly, electronic fingerprint sensing systems are required to be compact, highly reliable and low in cost.
Electronic fingerprint sensing using optical methods is well established. A simple lens-based video camera system is disclosed in U.S. Pat. No. 4,525,859 issued Jun. 25, 1985 to Bowles et al. More sophisticated units using holographic elements that project a corrected two-dimensional image onto a CCD imaging device are disclosed in U.S. Pat. No. 5,109,427 issued Apr. 28, 1992 to Yang.
More recently, electronic fingerprint sensors incorporating an array of electrodes to create a contour map of finger ridge capacitances have become popular. U.S. Pat. No. 4,353,056 issued Oct. 5, 1982 Tsikos discloses a sensor that has a two-dimensional row and column array of capacitors, each including a pair of spaced electrodes with sensing electronics, all overlaid with an insulating film. The sensor relies on the finger ridges to deform a pattern in the insulating film, thereby changing the underlying capacitance detected by the electrode array. This approach, however, requires extraordinary resiliency and durability in the polymer insulating film, which is difficult to achieve.
A more direct approach is disclosed in U.S. Pat. No. 5,325,442 issued Jun. 28, 1994 to Knapp, which discloses a two-dimensional electrode array that forms capacitors between each electrode on the substrate and a grounded finger that is placed in close proximity to the electrode. Thin film transistors located on an insulator are configured as switching elements to scan each row and column in the array. Additional circuitry measures the charging currents for each capacitor in the array as they are scanned, from which the individual capacitances are determined. These values are then used to create a two-dimensional capacitance map of the finger ridge patterns, which closely resembles the physical structure of the finger.
U.S. Pat. No. 6,016,355 issued Jan. 18, 2000 to Dickinson et al also proposes a two-dimensional matrix of electrodes on a substrate. The electrodes form an array of capacitors to a grounded finger in close proximity. This approach determines the amount of capacitance by placing a fixed voltage on each capacitor in the array and then measuring the time to discharge the capacitor with a constant current source in parallel.
All of the two-dimensional capacitive array approaches have disadvantages. First is the large number of transistor devices that are required to scan and measure the more than ten thousand capacitors in such an array at a 100 micron pitch. Even if the die area could be reduced by smaller device geometries, a large die size of at least 10 mm on a side is required to obtain sufficient contact area. Second is the problem of electrostatic discharge from a charged human body through the finger, breaking down the thin insulator that separates the finger from the sensitive electrodes and destroying the low voltage transistors that operate the array. A third problem is the risk of mechanical damage that is posed by direct physical contact of the finger to an exposed silicon die.
A fingerprint optical input apparatus including a contact image sensor for viewing a moving finger and providing an image is disclosed in U.S. Pat. No. 6,259,108 issued Jul. 10, 2001 to Antonelli et al. A linear sensor imaging method and apparatus for capturing an image of an object which moves, at an unknown variable or constant speed, past one or more linear sensor arrays is disclosed in U.S. Pat. No. 6,002,815 issued Dec. 14, 1999 Immega et al.
All of the known prior art electronic fingerprint sensing systems have had one or more disadvantages, including low reliability and high cost. Accordingly, there is a need for new and improved electronic fingerprint sensing systems and methods.